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When frequency is exceeded and enters to high frequency range the reaction of device and PCB object is changed. In high frequency the traces is not just a conductor with constant low resistance.
It will be a RLC. it means, traces act as inductor & capacitor. Due to the capacitance & inductance is depending on frequency too.
Xc=1/2*pi*f*C and Xl=2*pi*f*L
Xc: capacitive reactance
Xl: Inductive reactance
c: trace capacitance
l: trace inductance
for Exp.: f= 250 MHz, L=50 nH ==> Xl=78.5 OHM !!
As you see, the trace is not just a conductor in high frequency.
It was just one of the phenomenons on the high frequency range! There are several undesirable phenomenons will be happen. They must be eliminated by designers.
So it is vital to consider several guide lines to make a PCB that works in high frequency range safely & reliably.
This guide lines have 3 main duties:
1- Make a PCB that works smoothly!
2- Signal integrity
3- Meet EMC directive